Many metals such as aluminum, copper, chromium, zinc and tin are coated onto various substrates such as metal, glass and plastic by a vacuum deposition process in which a container is heated by electric resistance heating to vaporize metal fed into contact with the container. The container, which is commonly referred to as a "boat", is connected in an electric circuit in a series circuit relationship so that current flows directly through the boat, which in turn heats the metal in contact with the boat until it vaporizes. The metal is vaporized in an evacuated atmosphere for coating a product which may be individually introduced into the evacuated chamber or continuously fed through the chamber. Discreet products may include a television picture tube, an automobile head light, a toy or the like.
Presently, most resistance heated containers are composed of an intermetalic ceramic composite of titanium diboride and boron nitride alone or in combination with aluminum nitride. The composite has a very short lifetime and requires continual adjustment of the power supply. Moreover, the resistance characteristics of such heaters are not stable during operation since the metal component of the ceramic composite is a conductor which forms part of the electrical circuit. As a result metal vaporization is not uniform resulting in a non-uniform metal deposition.
A container of graphite coated with pyrolytic boron nitride has been suggested for use as an alternative to a resistance heater composed of an intermetallic ceramic composite. The coating of pyrolytic boron nitride is intended to electrically isolate the molten metal in the container from the current path through the graphite body and to supply more uniform heat to the metal. It has instead been discovered that the molten metal after only a short time interval will pass through the layer planes of the pyrolytic boron nitride coating and directly infiltrate the porous graphite body. Changes in the thermal cycle causes the boron nitride coating to crack within a short operating time interval. This destroys the utility of the graphite for use as a metal evaporator. Increasing the thickness of the pyrolytic boron nitride coating can delay, but will not significantly prevent leakage through the boron nitride coating.